feat: implement BrowserWindow.moveTop on X11 (#16629)

It was implemented on Mac and Win but not on X11.
Tested on Ubuntu 16.04 and 18.04.

Also added a unit test in spec/api-browser-window-spec.js.
This test BrowserWindow.moveTop verifies that calling moveTop
on a window does not give the focus to this window.

notes: BrowserWindow.moveTop is now available on Linux/x11

https://github.com/electron/electron/issues/12516

atom/browser/native_window_views.h

@@ -74,9 +74,7 @@ class NativeWindowViews : public NativeWindow,
   void SetContentSizeConstraints(
       const extensions::SizeConstraints& size_constraints) override;
   void SetResizable(bool resizable) override;
-#if defined(OS_WIN)
   void MoveTop() override;
-#endif
   bool IsResizable() override;
   void SetMovable(bool movable) override;
   bool IsMovable() override;